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Monitoring the Brain’s Memory-Making Cells

The brain cells that fire when a person watches a brief film clip are triggered again when the person thinks back on that imagery a few minutes later, a new study shows. The research offers insights into how the brain summons up past experiences and may also provide clues to brain disorders, like Alzheimer's disease, that harm short-term memory.

Scientists have known for decades that memories are often processed
and retrieved by the hippocampus, a curved structure deep in
the brain. But exactly how memories are recorded and recalled
remains a mystery.

Dr. Itzhak Fried, a professor of neurosurgery at the University
of California, Los Angeles, has been exploring the mechanisms
of memory at the single-neuron level in the human brain. For
the past 10 years, his research has focused on patients with
severe epilepsy who have many tiny electrodes implanted in their
brains. The electrodes are used to pinpoint seizure-causing brain
regions for surgical removal, but they can also provide information
about how individual brain cells process memories.

In his latest study, published in the online edition of Science on
September 4, 2008, Dr. Fried and colleagues at the Weizmann Institute
of Science in Israel monitored electrode recordings of hundreds
of neurons while patients watched a series of film clips and
then later recalled them. Thirteen patients participated in a
total of 43 viewing and recall sessions. The study was funded
in part by NIH's National Institute of Neurological Disorders
and Stroke (NINDS).

The patients were shown about a dozen brief clips, each lasting
about 5 to 10 seconds, of famous people, characters, landmarks
or activities. The clips included segments of TV shows like The
Simpsons and Seinfeld, portions of a speech by
Martin Luther King, the Golden Gate Bridge and the famous Hollywood
sign. Each clip was replayed a few times during each viewing
session. After viewing sessions, the patients engaged in a different
task for a few minutes. They were then asked to think about the
video clips they'd seen and say aloud which clips came to mind.

During the videos, more than half of the monitored neurons had
a significant response to one or more clips, and these neurons
became reactivated each time those clips were replayed. Later,
as the clips were remembered, the same neurons fired again. In
general, there was a 2-second lag between neuron firing and verbal
reporting of remembered clips.

For example, a neuron located near the hippocampus in one patient
showed a powerful response each time a clip from The Simpsons appeared.
It fired with less intensity when Seinfeld was shown
and remained essentially silent during 46 other clips. When the
patient later thought about the Simpsons clip, the neuron
fired for several seconds but had little or no response to other
memories. In all the patients, several neurons in and around
the hippocampus showed similar, selective responses to different
film clips.

It's not clear exactly what aspects of the clips triggered the
cells’ responses. Nevertheless, this study adds to a growing
body of evidence that neurons in the brain's memory centers can
play a dual role, responding first to sensory input and then
re-activating when that experience is later remembered.